Reflective polarizers are commonly used to enhance the brightness of liquid crystal (LC) displays and display systems. The LC display system typically includes an LC panel, behind which is an illumination assembly or backlight positioned to provide light to the LC panel. Brightness enhancement is provided by the reflective polarizer as the result of a light recycling process: light that cannot (because of its polarization state) contribute to the display output is reflected by the reflective polarizer back into the backlight, where some of the light is re-reflected towards the reflective polarizer in a different polarization state that can contribute to the display output and that passes through the reflective polarizer toward the user or viewer.
The LC panel includes a layer of liquid crystal material disposed between glass panel plates. Furthermore, the LC panel is sandwiched between two absorbing polarizer films: a front absorbing polarizer, attached to the front glass plate of the LC panel, and a back absorbing polarizer, attached to the back glass plate. The brightness-enhancing reflective polarizer is placed somewhere behind the LC panel, and behind the back absorbing polarizer.
In practice, design details of the reflective polarizer have an impact on exactly where the reflective polarizer can be placed in the display system to provide optimal, or at least acceptable, optical performance. Some types of reflective polarizers can be laminated directly to the exposed rear surface of the back absorbing polarizer. Those of ordinary skill in the art consider it necessary for these types of reflective polarizers to have a very low perceived color for the pass state of polarization both at normal incidence (light propagating along the optical axis of the display system) and at highly oblique incidence. Since the reflective polarizer is attached to the back absorbing polarizer, and the back absorbing polarizer is in turn commonly attached to the back glass plate of the LC panel, this is referred to as an “on-glass” configuration of the reflective polarizer. One reflective polarizer currently used in the on-glass configuration is a parabolically-stretched reflective polarizer, discussed further below. Another reflective polarizer used in the on-glass configuration is a multi-packet reflective polarizer, also discussed below.
Other types of reflective polarizers, now considered by those of ordinary skill in the art to have excessive perceived color for the pass state of polarization for obliquely incident light, are not laminated to the back absorbing polarizer of the display because the (undesirable) color associated with the reflective polarizer would be visible to the user through the absorbing polarizer and through the LC display. Instead, these latter types of reflective polarizers—multilayer optical film reflective polarizers of alternating polymer layers in which there is only one packet of microlayers, the microlayer packet having a thickness gradient or profile to provide broadband reflection, the multilayer optical film having been oriented using a standard tenter such that birefringent layers of the film are biaxially birefringent, such films referred to herein as Tentered-One-Packet (“TOP”) films or TOP reflective polarizers—are used in the display system as a stand-alone film, separated from the back absorbing polarizer by at least one air gap, and attached to a light diffusing film or layer that is disposed between the reflective polarizer and the back absorbing polarizer. The light diffusing layer has a significant haze value so as to effectively combine light rays that pass through the reflective polarizer in different directions, to reduce or eliminate the color associated with the TOP reflective polarizer from the standpoint of the user or viewer.
U.S. Pat. No. 7,791,687 (Weber et al.) appears to go against this prevailing opinion by disclosing embodiments in which a display panel has on one side thereof the combination of a first absorbing polarizer and a TOP reflective polarizer, these two polarizers being aligned with each other, and on the other side of the display panel is a second absorbing polarizer that is crossed with (oriented at 90 degrees relative to) the first absorbing polarizer. However, the '687 Weber patent refers to special cases in which the first absorbing polarizer is a low contrast absorbing polarizer (see e.g. column 2, lines 1-15, and column 3, lines 22-39). In the examples, the first absorbing polarizer has a contrast ratio of only about 5 (see e.g. Example 2, where the block state transmission of the first absorbing polarizer is reported as 20%). The '687 Weber patent says that in these cases where the first absorbing polarizer is of low contrast, the optical properties of the reflective polarizer become more important for maintaining the contrast of the display (see col. 3, lines 22-39). As demonstrated in the examples, the '687 Weber patent then assesses the display contrast by evaluating the block state (dark state) performance of the display. That is, the patent calculates and compares the spectral transmission through crossed polarizer systems, in which a combination TOP reflective polarizer/first absorbing polarizer (of low contrast, and aligned with the TOP polarizer) is crossed with a second absorbing polarizer of high contrast. These transmission spectra are calculated for various oblique polar angles θ and an azimuthal angle φ of 45 degrees. The calculated transmission through such crossed polarizer systems is representative of the dark state of the display, and is thus very low-all of the examples have transmissions under 4%, and some are well under 1%, over the entire visible wavelength region for the angles that were tested. The examples compare systems in which the TOP reflective polarizer is oriented different ways—some where the thickness profile of the TOP reflective polarizer is oriented one way, and some where it is oriented the opposite way-by comparing their calculated transmission spectra. This analysis led the '687 Weber researchers to conclude the thickness profile of the TOP reflective polarizer should be oriented such that a majority of the layers having a smaller optical thickness are disposed closer to the display panel than the layers having a larger optical thickness. In embodiments where the combination TOP reflective polarizer/low contrast absorbing polarizer is disposed behind (rather than in front of) the display panel, this means that the thickness profile of the TOP polarizer should be oriented so that thinner layers face the front, i.e., towards the user, and the thicker layers face the back, i.e., away from the user and towards the backlight.